Novel precipitated siliceous products and methods for their use and production

ABSTRACT

A method for producing a precipitated silicon dioxide having a new combination of physical and chemical properties is disclosed. The pigments are produced by acidulating a solution of an alkali metal silicate with an acid under controlled precipitation conditions. The aqueous reaction medium comprising the precipitated silica is then post-conditioned by introducing a second silicate solution into the reaction vessel and thereafter adding additional acid to react with the said second silicate solution. By varying the amount of the silicate employed in the post-conditioning step, a product is obtained which has a unique combination of physical and chemical properties including reduced wet cake moisture content, high surface areas and oil absorptions, improved surface activity, friability, wetting characteristics, and the like. The product has particular utility for use as a rubber reinforcing agent because of its increased surface activity and oil absorption, etc. The product, however, may be used in paints, paper, detergents, dentifrice compositions, molecular sieves, and polymeric compositions. An unexpected discovery of the invention involves the production of a rubber reinforcing silica which has a decreased wet cake moisture content. In one particularly advantageous embodiment, an adduct material, such as aluminum, is added to control the refractive index and surface area of the product.

This is a continuation of application Ser. No. 876,284, filed Feb. 9,1978 which is a continuation of Ser. No. 557,707, filed Mar. 12, 1975,both now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to novel synthetic precipitated silicasand to a process for producing a synthetic precipitated silicon dioxidehaving a new and improved combination of properties and characteristics.More particularly, the invention relates to the production ofprecipitated high structure silicas produced by reacting aqueous alkalimetal silicate solutions with an acidification agent. The novel productsare high structure finely divided silicas having certain uniqueproperties particularly with respect to structure index, oil absorption,void volume, surface activity, friability, wetting characteristics, andfurther properties. The novel products are especially suitable for useas reinforcing agents in rubber, in paints, paper, detergents, molecularsieves, and in polymeric compositions.

2. Description of the Prior Art

As known in the art, finely divided silica or silicon dioxideparticulates can be prepared by the acidulation of an aqueous silicatesolution with an acid, such as sulfuric acid. Such products arecommercially available and are characterized by, and have, the followingproperties: high structure, high wet cake moisture content, high oilabsorption, low valley abrasion, high surface area, and low packdensity. Because of properties such as high oil absorption, the pigmentshave been successfully used as reinforcing pigments in rubber. However,the high wet cake moisture content is disadvantageous in that the dryingand filtration rates are increased. Further, the aforementionedproperties of said known and commercially available silicas render themunsuitable for many uses.

In this regard and generally speaking for the moment, known processesincluding those described in the literature as well as those techniquesemployed in the industry produce products suitable for use asreinforcing fillers. Thus, in U.S. Pat. No. 2,940,830 which issued June14, 1960 to F. S. Thornhill, there is described a process for preparingfinely divided silicas which are suitable as reinforcing agents inrubber compositions. Thornhill more specifically describes a process ofpreparing a silica material which is characterized by having an averageultimate particle size of 0.015 to 0.04 micron and a surface area of 25to 200 square meters per gram by the controlled rate of addition of acidto an alkali metal silicate wherein the resultant slurry is constantlymaintained at a pH above 7 in order to achieve the aforementionedend-product characteristics. The Thornhill patent is specificallydirected to the production of a product suitable as a reinforcing agentin rubber compositions.

In U.S. Pat. No. 3,235,331, which issued Feb. 15, 1966 to Nauroth etal., there is described a process for producing a precipitated silicawhich is also stated to be useful as a reinforcing agent for rubber.More specifically, this patent discloses a process wherein an aqueousalkali metal silicate solution and acid are simultaneously added to areaction vessel. In the Nauroth et al. patent, it is pointed out thatthis simultaneous addition is continued until the viscosity of the poolrises through a maximum and then falls to a substantially lower value.The amount of the acidification agent and the alkali metal silicate areproportioned so as to maintain the pH of the resulting slurrysubstantially constant throughout the major portion of the reaction andin the range of about 10 to 12. The process is generally conducted at atemperature of 80° to 90° C. and the end product, after drying, resultsin a silica which may have a surface area of 260 square meters per gram.The patentees point out that the product is satisfactory as areinforcing agent for rubber.

In U.S. Pat. No. 3,445,189 issued May 20, 1969 to Maat et al., there isdescribed a process for producing finely divided silicic acid bysimultaneously adding solutions of an alkali silicate and a strongmineral acid to water at a temperature between 70° C. and 90° C. whilemaintaining the reaction pH between 7 and 9. The patentees point outthat the product obtained by the aforementioned process is a finelydivided non-gelatinous silicic acid which is useful as a filler fornatural and synthetic rubber and other elastomers. It is also disclosedin this patent that for a silica to be useful as a filler for naturaland synthetic rubber and other elastomers, its surface area and oilabsorption are of vital importance. This patent further discloses thatextensive investigation have further indicated that if a finely dividedsilicic acid is to have good reinforcing properties for rubber, it musthave a surface area of 100 to 250 m² /g and an oil absorption of morethan 2 cc/g or 200 cc/100 g. See column 2, lines 18 through 22.

In U.S. Pat. No. 3,730,749, which issued May 1, 1973 to James E. Morgan,there is disclosed a process for preparing silica for use in reinforcingcompositions. It is pointed out in Morgan that the viscosity increasewhich occurs during the acidification or neutralization of aqueousalkali metal silicate is substantially minimized by adding a controlledamount of an alkali metal silicate. In Examples I, II, and III of thispatent, it is also noted that the silica filter cakes had solid contentsof 18.5; 24.9; and 25.1 percent, respectively. This means that thepercent wet cake moisture of the silicas disclosed in Examples I, II,and III is one hundred minus the percent solid content in the filtercake. In other words, the percent wet cake moisture (% WCM) of silicasmentioned in Examples I, II, and III is 81.5; 75.1; and 74.9,respectively. The surface area, the average ultimate particle sizes, andrubber data of silicas produced by the teachings of Examples II and IIIare listed in Table 3 which also sets forth that rubber compositionsincorporating the silicas of Examples II and III have desirable rubberproperties. It is further substantiated by this patent that rubberproperties of silicas are related to the high wet cake moisture of thesilica pigment. Thus, it is taught that a silica of high percent wetcake moisture and suitable particle size and surface area has betterrubber properties than the corresponding material of lower wet cakemoisture. Thus, the silicas disclosed in Morgan have a higher structureindex, and therefore the silicas are useful rubber reinforcing fillers.

From the above it will be seen that the structure index of a silica isrelated to the rubber properties--a silica of higher structure indexwill have better rubber properties than a silica of lower structureindex. At this point and before turning to the remarkable concept of thepresent invention, the various types of synthetic silicas, as well as"structure" and "structure index" should therefore be discussed.

In this regard, and as known and accepted in the art, commerciallyavailable synthetic silicas are derived either by a liquid phase or avapor process. Silicas obtained by the vapor process are called fumed orpyrogenic silicas. Products obtained by the liquid process arecategorized as silica gels and precipitated silicas. Thus, there arethree distinct types of synthetic silicas on the market:

1. Pyrogenic Silicas

Pyrogenic or fumed silicas are prepared by reacting silicontetrachloride vapor with oxygen and hydrogen gas at high temperatures.These products have high external surface areas and differ from othersilicas (e.g., gels, precipitated silicas) prepared from the liquidphase process. Cabot and DeGussa are two suppliers of pyrogenic silicas.

2. Silica Gels

Silica gels are of two types: hydrogels and aerogels. Hydrogels areprepared by reacting a soluble silicate such as sodium silicate withstrong sulfuric acid. The gel is washed salt free, dried, steammicronized, and then classified. Aerogels are prepared from crudehydrogels by displacing its water content with an alcohol. The alcoholis then recovered by heating the gel in an autoclave.

Aerogels are lighter and fluffier than hydrogels because the shrinkageof the gel structure is avoided during the drying process. Gels havevery large surface areas, generally in the range of 300-1,000 m² /g andhigh porosities. Silica gels are offered, e.g., by W. R. Grace andCompany under the trademark "Syloid"; by Monsanto, under the trademark"Santocel"; and by Glidden, under the trademark "Silcron."

3. Precipitated Silicas

Precipitated silicas are produced by the de-stabilization andprecipitation of silica from soluble silicate by the addition of amineral acid and/or acidic gases. The reactants thus include an alkalimetal silicate and a mineral acid, such as sulfuric acid or anacidulating agent such as CO₂.

When the acidification agent is added to the alkali metal silicate at acertain point during the process, the silica starts precipitating. Theaddition of the acidification agent is continued until the M₂ O of thealkali metal silicate (M being the alkali metal) of the ultimate silicais less than about 1% by weight. Thus, as a general rule, theacidification agent is added to the alkali metal silicate to neutralizethe alkali portion bound to the silicate anion. The reaction slurry isfiltered and washed free of reaction by-product, which is the alkalimetal salt of the acidification agent. The filter cake is dried andmilled to obtain a silica of desired degree of fineness.

Prior to the drying step the silica filter cake generally results in afilter cake which contains a surprisingly high amount of water. Forexample, a silica which is useful as a filler for reinforcement ofrubber and elastomers generally contains 80% to 85% water in its cake.For example, see Example No. I, U.S. Pat. No. 3,730,749 where thepercent wet cake moisture is 81.5. The percent water present in thefilter cake is known as percent wet cake moisture or generallyabbreviated as "% WCM." One hundred minus the % WCM gives the solidcontent of the filter cake, i.e., the amount of silica which can berecovered in the solid form upon drying the filter cake. The percentsolid content of the filter cake is termed percent filter cake solidsand generally abbreviated as "% FCS." Thus, % WCM and % FCS are relatedby the equation:

    % WCM=100-% FCS

or

    % FCS=100-% WCM

If we know the value of % WCM, we can calculate % FCS or vice versa.

Thus, a silica filter cake having 85% WCM will have 100 minus 85 or 15%FCS. This means that 15 pounds of silica can be recovered from such afilter cake by evaporating or drying 85 pounds of water from 100 poundsof filter cake. The total weight of filter cake consists of water andsolid silica. In the example where the % WCM is 85, one can recover only15 pounds of solid silica as can be seen below: ##EQU1##

Thus, there are 85 pounds of water associated with 15 pounds of solidsilica content or (85/15)×100=567 of water per 100 pounds of solidsilica.

The water associated with the silica content of filter cake isstructural water. This water is present whereby it occupies theavailable space between the silica aggregates and also the space insidethe silica aggregates. As used herein, the term "structure" is definedas the ability of a silica to hold water in its wet cake. When silicas,such as the aforementioned known prior art products, hold a highpercentage of water, i.e., from about 70 to 85%, they are known as highstructure silicas. Materials holding less than 70% or from about 50 to70% are referred to as low structure silicas. This total structuralwater content is a very important property of silica and is directlyrelated to the functional and end use properties of silica. The amountof total structural water associated with 100 pounds of solid silicacontent of the filter cake is defined as "structure index" andabbreviated as S. I.

Mathematically, structural index (S. I.) of silica can be calculated ifeither the % wet cake moisture (WCM) or the % filter cake solid (FCS)values of said silica are known:

    S. I.=(% WCM)/(100-% WCM)×100=% WCM/% FCS×100

Structural index of silicas in wet cake moisture range of 80-85% islisted in Table I.

                  TABLE I                                                         ______________________________________                                        Structure Index of Silicas With                                               % WCM of 85-80                                                                % WCM       100 - % WCM      S. I.                                            ______________________________________                                        85          15               567                                              84          16               525                                              83          17               488                                              82          18               455                                              81          19               426                                              80          20               400                                              ______________________________________                                    

Prior art precipitated silicas such as disclosed in the aforementionedpatents (see U.S. Pat. Nos. 2,940,830; 3,235,331; 3,445,189; 3,730,749)are high structure silicas having high S. I. values. As stated, thesesilicas are useful as reinforcing fillers in elastomers and rubber. As afinal point in this discussion of the prior art, it may be noted that inaddition to the aforementioned high structure reinforcing silicas, therehas recently been developed a new class of low structure silicacompounds which, though unsuitable as reinforcing fillers, have utilityfor use in specific areas such as polishing and cleaning agents indentifrices. Specific examples of such new products and methods fortheir production are disclosed in U.S. patent applications Ser. Nos.286,655, filed Sept. 6, 1972 (now allowed) and 472,580, filed May 22,1974.

SUMMARY OF THE INVENTION

In summary, the present invention relates to novel precipitated highstructure silicas, to a unique process for producing same and to methodsfor their use. The present invention is based, in part, on theremarkable discovery that the post-conditioning of suspended finelydivided silica particulates, under conditions as described in detailhereinafter, not only affects or significantly alters their properties,but also serves to enhance their properties for a given use, i.e., asrubber reinforcing agents and to render them suitable for use (becauseof said new properties) in new fields.

In its broadest aspects, in the practice of the method of the instantinvention, a known volume of an aqueous solution of an alkali metalsilicate is first introduced into a reaction vessel. An acidificationagent, such as sulfuric acid, is then added to the silicate solutionuntil finely divided silica or silicon dioxide particulates areprecipitated. The amount of the acidification agent added should be thattheoretically required to react with the silicate to form, orprecipitate, silicon dioxide in accordance with the general formula:

    M.sub.2 O(SiO.sub.2).sub.x +H.sup.+ A.sup.- →(SiO.sub.2).sub.x.H.sub.2 O+MA

wherein M is an alkali metal, A is an acid salt radical, and x is anumber from 1 to 4. During the precipitation, sufficient agitation isprovided to insure intimate mixing of the reactants.

As briefly stated above, the present invention embodies the concept anddiscovery that the properties of the aforesaid precipitated silicondioxide particulates may be significantly altered and/or tailored to agiven end use by introducing a second and known volume of an alkalimetal silicate solution into the reaction mass comprising theprecipitated silicon dioxide particulates and acidifying the thus addedsilicate so that, at least to the theory best understood by theinventors, a fine coating of silicone dioxide is formed on the precursorparticulates. Finely divided silica products produced in this manner,have, depending upon the amount of the post-conditioning, enhancedreinforcing rubber properties and a unique overall combination ofproperties which render them particularly suitable for use in manyareas. In addition, the precipitated silicas have a reduced wet cakemoisture content which significantly reduces their overall cost ofproduction. The fact that a product can be produced with improved rubberreinforcing properties, yet have a lower wet cake moisture content, isremarkable and is in direct contrast to the teaching of the prior art.In other words, the findings of the invention are unexpected. Inaddition, new use areas are made possible because of the aforesaid newproperties. Thus, the precipitated pigments produced in accordance withthe invention result in silica products which have a unique balance ofphysical, chemical properties as compared to conventionally knownsilicas.

It is accordingly a general object of the present invention to providefinely divided silicon dioxide particulates having a new and uniquecombination of physical and chemical properties.

Other objects are to provide a process for controlling the relativesizes and uniformity of primary and secondary particles of silica forthe production of improved silica pigments for various applications; toprovide a silica pigment of minimal or no built-in microporosity; toprovide a silica pigment of improved surface reactivity, friability,wetting characteristics and generally improved physical/chemicalproperties; to decrease the structure index and wet cake moisture ofsilica pigment without corresponding reduction in absorption, withoutloss of thickening and without loss of viscosity buildingcharacteristics of the silica pigment; to provide an improved process toenhance the filtration characteristics of silica pigment; to provide asilica pigment of improved end-use functionality and to provide animproved process for producing silica pigments of equal or betterfunctionality than prior art silicas, but at an economical cost.Further, objects of the present invention are to increase theparticle-particle void structure without increasing the overallproduction cost of silica pigments. Yet another object of the presentinvention is to provide a process for producing high structure finelydivided silicon dioxide by the acidulation of an alkali metal silicate,the improvement comprising post-conditioning the precipitated silicareaction slurry with an alkali metal silicate solution to decrease thestructural microporosity. The microporosity is disadvantageous to havefor the effective usefulness of the pigmentary silica in rubbercompositions. The silicate treatment of the instant process promotes thegrowth of particles and results in a homogeneous final product ofuniform particle size wherein structural microporosity is avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

The manner in which the foregoing and other objects are achieved inaccordance with the present invention will be better understood in viewof the following-detailed description and accompanying drawings, whichform a part of the specification and wherein:

FIG. 1 is a graph illustrating the change in the structure index ofsilica as a function of silicate post-treatment.

FIG. 2 is a graph illustrating the change in the oil absorption ofsilica as a function of silicate post-treatment.

FIG. 3 is a graph illustrating the change in the void volume of silicaand function of silicate post-treatment.

FIG. 4 is a graph illustrating the change in the surface area of silicaas a function of silicate post-treatment.

FIG. 5 is a graph illustrating the percent change in friability ofsilica as a function of silicate post-treatment.

FIG. 6 is a graph illustrating how the structure index of the controlledrefractive index silicas changes as a function of the silicatepost-treatment.

FIG. 7 is a graph illustrating how the oil absorption of the controlledrefractive index silicas changes as a function of the silicatepost-treatment.

FIG. 8 is a graph illustrating how the void volume of the controlledrefractive index silicas changes as a function of the silicatepost-treatment.

FIG. 9 is a graph illustrating how the surface area of the controlledrefractive index silicas changes as a function of the silicatepost-treatment.

FIG. 10 is a graph illustrating how the percent friability of thecontrolled refractive index silica changes as a function of the silicatepost-treatment.

FIG. 11 is a graph illustrating the change in modulus values of rubbercompositions comprising silica pigments of Examples I through V as afunction of the percent silicate post-treatment.

FIG. 12 is a microphotograph of the product of the invention with 10%post-conditioning, no adduct.

FIG. 13 is a microphotograph of the product of the invention with 30%post-conditioning, no adduct.

FIG. 14 is a microphotograph of the product of the invention with 70%post-conditioning, no adduct.

FIG. 15 is a microphotograph of the product of the invention with 20%post-treatment, with adduct, aluminum sulfate.

FIG. 16 is a graph illustrating a product of the prior art, nopost-conditioning, acidulation of sodium silicate with sulfuric acid.

DESCRIPTION OF PREFERRED EMBODIMENT(S)

In accordance with the present invention, there is provided an improvedprocess for producing a high structure silica which is useful as areinforcing agent in rubber and elastomers and, in fact, has improvedproperties therefor. In the practice of the invention, an alkali metalsilicate solution is first acidulated until at least 99% of the silicondioxide is precipitated. That is, 99% of the alkali portion bound to thesilicate anion has been reacted. At this point, the precipitated silicaslurry is post-conditioned with calculated amounts of a silicatesolution. Acid is then introduced into the reaction medium in an amounttheoretically required to react with the second silicate solution addedto the precipitated silica slurry. The amount of the silicate solutionadded in the post-conditioning step will vary depending upon theparticular properties desired, but should be from about 10 to 70% byweight of the initial silicate that is acidulated with the acid. Stateddifferently, the silicate and the acid introduced in thepost-conditioning step should be such that it precipitates additionalsilica, the latter comprising from about 10 to 70% of the silica presentin the slurry prior to the said post treatment.

In the prior art, it is generally taught that during the acidulation ofalkali metal silicate, a maximum reaction viscosity is observed when thealkali metal silicate is neutralized between about 25 percent and 60percent (theoretical). It is suggested that the increase in viscositycontributes to the formation of aggregates having a wide range ofparticle sizes which are unsuitable for use in rubber and papercompositions. It is also generally recognized that due to localizedgelation, the proper nucleation of silica micelles does not take placeduring the periods of high viscosity. This results in and accounts forvery high wet cake moisture content of prior art silicas. By the presentinvention, it is possible to reduce the wet cake moisture ofprecipitated silicas without sacrificing the beneficial rubberproperties. In fact, they are improved. It is also taught that the wetcake moisture and the structure of silica go hand in hand. Thus, it isdisclosed that as the wet cake moisture goes up, the rubber propertiesof a pigment are also improved. The increase in wet cake moisture isrelated to an increase in production costs. Higher wet cake moisturemeans that during factory processing, the drying and processing costs ofsuch a silica go up. It is, therefore, of vital importance to reduce thewet cake moisture of a silica pigment while still maintainingsubstantially high structure index in a silica pigment for it tofunction as a useful reinforcing pigment. This was not possible prior tothe present invention.

Thus, a focal point of the present invention is to provide an improvedprocess for producing a silica pigment which is useful as a rubberreinforcing filler. The improved process results in the reduction of wetcake moisture, yet it does not degrade the rubber reinforcing propertiesand other utility of silica pigments. This, as stated, is trulyunexpected.

In the practice of the present invention, the following process stepsare employed.

(1) A known volume of an alkali metal silicate (of known or fixedcomposition) is charged to the reactor.

(2) The acidification agent is added gradually to the silicate solutionuntil at least 99% of the theoretical amount of the silica isprecipitated.

(3) The reaction temperature is maintained between 60° C. to 95° C.throughout the entire process of acidification.

(4) The reaction slurry is conditioned by post-treating the precipitatedsilica with controlled amounts of alkali metal silicate solution.

(5) An acid is then added in an amount sufficient to react with thesilicate introduced in Step (4).

(6) The reaction mass is adjusted to a desired final pH depending on itsintended use.

(7) After post-treatment with silicate solution and final pH adjustment,the reaction slurry is filtered, washed free of reaction by-product,dried and may be milled to the desired degree of fineness.

It was unexpectedly discovered that the post-treatment of silicareaction slurry results in significant reduction in the wet cakemoisture content of the silica product without corresponding andexpected reduction in end-use functionality. In fact, rubber and otherproperties are improved.

The alkali metal silicate used should normally have the composition M₂ O(SiO₂)_(X) where M is an alkali metal and X is 2 or above, usually 2 to4 including the fractional numbers. Sodium or potassium silicates andother alkali metal silicates can be used, but sodium silicate ispreferred because it is most economical to use. Several mineral acidsand other weak acidification agents like CO₂ and organic acids may beemployed in the practice of the invention. Examples of mineral acidswhich have been found especially suitable include phosphoric, nitric,hydrochloric, and sulfuric acid. Of these, hydrochloric acid andsulfuric acid are preferred and sulfuric acid is particularly preferredbecause it is the most economical mineral acid to use. If the mineralacid is a dibasic acid, i.e., sulfuric, the concentration of the acidsolution should be on the order of 8 to 22% and preferably from about 10to 15% by weight. Other acids, such as mono or tribasic acids, shouldhave their concentration adjusted to normalities (n) equivalent to thedibasic acid. It is believed that the low mineral acid concentrationhelps to minimize localized reactions of high concentration of thealkali metal silicate solution.

Turning now to further specific details, in the practice of theinvention, the alkali metal silicate is first charged to a reactor as asolution thereof and the solution is heated to a temperature in therange of from about 60° C. to 90° C., preferably on the order of fromabout 60 to 80° C. with continuous agitation. The concentration of thealkali metal silicate should be in the range of from about 8 to 25% byweight silicate, preferably from about 8 to 15% by weight silicate.

The acidulating agent or acid, e.g., sulfuric acid, is next charged tothe reaction vessel until the precipitation is substantially complete,i.e., at least 99% of the theoretical amount of the silica isprecipitated. Following precipitation and while maintaining theprecipitate in suspension (e.g., by agitation), the slurry or suspensionis post-conditioned by first introducing an alkali metal silicatesolution having substantially the same concentration as the initialsilicate solution. The amount added should comprise from 10 to 70% byweight of that silicate solution initially charged to the reactor. Ifthe concentration of the solution is the same, the amount of the totalsilicate solution added in the post-treatment will, of course, simplycomprise from 10 to 70% of the solution added initially. Acid is thenadded to react with the second silicate added in the post-treatmentstep.

The instant invention results in a new class of products having a uniquecombination of physical and chemical properties. These include thecombination of an oil absorption of greater than 190 cc/100 grams and astructure index of 505. In fact, in the practice of the invention onecan obtain a product having an oil absorption in the range of frombetween about 190 to 212 cc/100 grams with a corresponding structureindex range of from between about 505 to 350. As will be seen in thefollowing examples, it was found that as the percent post-treatment wasincreased above 10%, the structure index decreased yet at the same timethe oil absorption increased. This was unexpected and the combination ofincreased oil absorption and reduced wet cake moisture (or structureindex) is novel. Further properties of the product of the inventioninclude a surface area in the range of from between about 120 to 220 sq.meters per gram; a void volume of from between about 3.55 to 4.44ccHg/gram SiO₂ and a friability of up to 98%.

In one particularly advantageous embodiment, the refractive index andsurface area of the precipitated product is controlled by the additionof an adduct element, such as aluminum, magnesium, and the like. In thisregard it may be noted that post-conditioning actually decreases thesurface area of the resulting product over that of a control (notreatment). A minimum surface area is obtained at a post-conditioningtreatment level of between about 30 to 50%. This reduction in surfacearea is believed to be due to the elimination of microporosity. However,the adduct not only serves to increase the refractive index, but alsoincreases the surface area of post-conditioned product. In thisembodiment, the acid is premixed with a solution of the adduct material,i.e., aluminum (preferably in the form of a water soluble salt thereof,such as aluminum sulfate, etc.) and the acid-metal salt mixture is thenused for acidulating the aqueous alkali metal silicate solution. It hasbeen found that the addition of the adduct changes (increases) thesurface area and refractive index of the product but does notsubstantially affect the other properties thereof. Specific metals thatmay be employed include water soluble salts of aluminum, magnesium,calcium, and zinc.

In the practice of the invention, improved and very important processingadvantages are also obtained. While particular embodiments have beendisclosed for illustrative purposes, the invention is not intended to belimited thereto. For example, a product can be easily produced for aspecial utility. Also, and as should be readily appreciated by thoseskilled in the art, no special equipment is required in the methodhearin described. In this regard, the reactor or reaction vessel shouldbe equipped with heating means, e.g., a steam jacket, in order tomaintain the desired reaction temperature and should have adequateagitation means to produce a strong backflow on the body of the liquidand to avoid zones of high concentration of the incoming reactants.Storage vessels (for the reactants) connected to the reaction vesselthrough lines fitted with flow control means may also be provided. Thereaction vessel may be equipped with an outlet line leading to a filterwhich may be of conventional design. As noted above, the filtered massis washed and dried. Such steps may also be conducted in conventionalequipment.

The following Examples will serve to further illustrate the presentinvention, but it should be expressly understood that they are notintended to limit it thereto.

EXAMPLE I

A concentrated sodium silicate solution of 38.2% solids and ofcomposition 10.7% Na₂ O and 27.5% SiO₂ was diluted with sufficient waterto prepare a dilute sodium silicate solution of 3.78% Na₂ O and 9.52%SiO₂. The specific gravity of this dilute silicate solution was 1.121.

A 93.0% commercial sulfuric acid from the storage tank was diluted withsufficient water to prepare a dilute sulfuric acid solution of 11.4%concentration and of specific gravity 1.076 @ 20° C. These dilute sodiumsilicate and dilute sulfuric acid solutions were used for preparingsilica pigments per following reaction conditions:

    ______________________________________                                        Sodium Silicate Volume   37.85 liters                                         Sodium Silicate Composition                                                                            3.78% Na.sub.2 O                                                              9.52% SiO.sub.2                                      Specific Gravity of Dilute Silicate Solution                                                           1.121 @ 20° C.                                Dilute Sulfuric Acid Concentration                                                                     11.4%                                                Temperature of Sulfuric Acid                                                                           38° C.                                        Sulfuric Acid Addition Rate                                                                            400 ml/min                                           Reaction Temperature     80° C.                                        ______________________________________                                    

PROCEDURE

37.85 liters of sodium silicate solution (of composition 3.78 percentNa₂ O, 9.52 percent SiO₂) was added into a 130 liter stainless steelreactor jacketed for steam heating.

The silicate solution was heated to a reaction temperature of 80° C.Sulfuric acid solution of 11.4% concentration and at 38° C. was added atthe rate of 400 ml/min to the reactor maintained at a reactiontemperature of 80° C.

The acid addition was continued until a final pH of 5.8-6.0 wasobtained. The reaction slurry was boiled at 100° C. for 20 minutes andthe final pH was readjusted to 5.8 to 6.0. The resulting slurrycontained 8% silica and was filtered on a filter press. The filter cakewas washed with water to free it from the reaction by-product (sodiumsulfate).

A portion of the filter cake was dried @105° C. until constant weight todetermine the percent wet cake moisture and the structure index ofsilica pigment.

The remainder of the silica was dried in the oven and dry materialmilled to the desired degree of fineness. The dry fine particulatesilica powder was subjected to various physical-chemical tests.

The above-mentioned reaction conditions and procedure are typical of amethod by which a conventional prior art precipitated silica isproduced. Therefore, this Example I will be compared to support theimproved properties of silica obtained by the instant invention.

The prior art silica of Example I was subjected to various tests and thefollowing data was obtained:

    ______________________________________                                        % Wet Cake Moisture                                                                            =     86.0                                                   Structure Index  =     615                                                    Oil Absorption   =     172 cc/100 gram                                        Particle-Particle Void Volume                                                                  =     2.95 cc Hg/gram silica                                 BET Surface Area =     164 square meters per gram                             Percent Friability                                                                             =     10                                                     ______________________________________                                    

The following procedures were used for calculating the above data.

The structure index (S. I.) was calculated by using the followingequation:

    S. I.=(% WCM/100-% WCM)×100

The oil absorption of the end product produced from Example I wasdetermined by the rub-out method. This method is based on a principle ofmixing linseed oil with a silica by rubbing with a spatula on a smoothsurface until a stiff putty-like paste is formed. By measuring thequantity of oil required to give a paste mixture which will curl whenspread out, one can calculate the oil absorption value of the silica--avalue which represents the volume of oil required per unit weight ofsilica to saturate the silica sorptive capacity. Calculation of oilabsorption value was done as follows: ##EQU2##

The specific surface area of the end product was determined by thenitrogen absorption method described by Brunauer, Emett, and Teller(BET) in the "Journal of the American Chemical Society," Volume 60, page309, published in 1938.

The particle-particle void volume of silica was determined by using theAminco-Winslow Porosimeter. This instrument is a completely hydraulicmachine used to measure the void structure of various materials. Themercury is forced into the voids as a function of pressure and thevolume of mercury displaced per gram of sample is calculated at eachpressure setting. Increments in volume (cc/g) at each pressure settingare plotted against the void size increments corresponding to thepressure setting increments. The following data was collected for theprior art, control silica of Example I (see Table IA).

                  TABLE IA                                                        ______________________________________                                        Pressure                                                                             Pore                                                                   Gage   Diameter Cumulative Intruded                                                                           Intruded                                      PSIG   Microns  Volume, cc Hg/g Silica                                                                        Volume, cc Hg/g                               ______________________________________                                               102      0.00            0.00                                                 51       0.05            0.05                                                 26       0.23            0.18                                          3.3    13       0.48            0.25                                          16.3   6        0.71            0.23                                          41     3        0.96            0.25                                          96     1.6      1.32            0.36                                          206    0.8      1.62            0.30                                          427    0.4      1.83            0.21                                          869    0.2      2.01            0.18                                          1750   0.1      2.18            0.17                                          3520   0.05     2.44            0.26                                          7060   0.025    2.95            0.51                                          ______________________________________                                    

The above data (see Table IA) suggests that void sizes between 0.025microns and 51 microns exist in the prior art control silica of ExampleI. The total particle-particle volume of the voids is 2.95 cc Hg/gramsilica. As will be seen later in Examples II through V that by using theimproved process of the instant invention, it is possible to increasethe total void volume to values higher than 2.95.

EXAMPLE II

The procedures of Example I were repeated except that the improvementconsisted in post-treating the precipitated silica reaction slurry withsilicate solution.

In the actual practice of this invention, 37.85 liters of sodiumsilicate solution (of composition 3.78% Na₂ O and 9.52% SiO₂) was addedinto a 130 liter stainless steel reactor jacketed for steam heating.

The silicate solution was heated to a reaction temperature of 80° C.Sulfuric acid of 11.4% concentration at 38° C. was added to the reactorat the rate of 400 ml/min. The reaction temperature was maintainedconstant at 80° C.

The acid addition was continued until substantially all the silica wasprecipitated. The precipitated reaction slurry at pH 7 was post-treatedwith silicate solution. The treatment level corresponded to 10% byweight of the precipitated silica in the reaction slurry.

In the actual practice of the invention, the reaction slurry containingthe precipitated silica at a neutral pH was post-treated by adding 3.785liters of sodium silicate solution in five minutes. The post-treatmentincreased the pH of the reaction medium. Additional sulfuric acid wasadded to bring the final pH of the reaction slurry below 6.0. Theresulting slurry was boiled at 100° C. for 20 minutes and filtered on afilter press. The filter cake was washed with water to free it from thereaction by-product (sodium sulfate).

A portion of the filter cake was dried at 105° C. until constant weightto determine the percent wet cake moisture and the structure index ofthe silica product.

The remainder of the silica was dried in the oven and dry materialmilled to the desired degree of fineness. The dry, fine particulatesilica powder was tested for various physical and chemical propertiesand the following data was obtained:

    ______________________________________                                        % Wet Cake Moisture  83.5                                                     Structure Index      505                                                      Oil Absorption       190 cc/100 gram                                          Particle-Particle Void Volume                                                                      3.55 cc Hg/g silica                                      BET Surface Area     141 m.sup.2 /g                                           Percent Friability   20                                                       ______________________________________                                    

Examining the above data and comparing with Example I, it will becomeclear that the post-treatment of the precipitated silica slurry resultsin higher void volume, higher oil absorption, better friability, lowerstructure index and lower surface area than the control (see Example I).

The actual mechanism of the improved properties obtained in the instantinvention is not clearly understood. It is, however, believed thatacidulation of silicate below a certain pH, preferably around neutralpH, results in a large number of nuclei. The post-treatment of silicaslurry with silicate solution results in the growing of particles andmaking the particles more uniform. Because of the uniformity of theparticles, the oil and the void volume increases. This also explains theincrease in friability.

The growth of particles results in a decrease of surface area. This isattributed to the dissolution of small particles and thenreprecipitation in the form of large particles. This then gives a silicaproduct of predominantly more uniform size than the prior art silica.Because of the loss of some nuclei in the post-treatment process, thesurface area decreases.

EXAMPLES III, IV AND V

The procedures of Example I were repeated except that the precipitatedsilica reaction slurry prior to filtration thereof was post-treated withsilicate solution which corresponded to the treatment level of 30%(Example III), 50% (Example IV), and 70% (Example V). The volume ofsilicate solution required for post-treatment silica slurry in ExamplesIII, IV, and V is as follows:

    ______________________________________                                                              Post-Treatment                                          Example No.                                                                             % Treatment Volume of Silicate Solution                             ______________________________________                                        III       30          11.35 liters                                            IV        50          18.92 liters                                            V         70          26.50 liters                                            ______________________________________                                    

Data obtained in Examples I through V is listed in Table II.

                                      TABLE II                                    __________________________________________________________________________    PROPERTIES OF SILICAS AS A FUNCTION OF POST-TREATMENT LEVEL                                                     BET                                         Example                                                                            % Treatment  Structure                                                                          Oil   Void Surface                                                                            Percent                                No.  Level  % WCM Index                                                                              Absorption                                                                          Volume                                                                             Area Friability                             __________________________________________________________________________    I     0     86.0  615  172   2.95 164  10                                     II   10     83.5  505  190   3.55 141  20                                     III  30     82.0  455  202   4.14 123  98                                     IV   50     81.9  452  200   4.07 120  60                                     V    70     79.8  395  197   4.05 153  40                                     __________________________________________________________________________

The following conclusions can be drawn by examining the data of TableII.

1. The treatment level, as it increases, results in the decrease ofstructure index.

2. The oil absorption increases with the increase in the treatmentlevel, and the oil absorption appears to go through a maximum at 30%treatment level.

3. The particle-particle void volume also follows the same trend as theoil absorption data. The void volume increases with increase in thetreatment level and appears to go through a maximum at about 30%treatment level.

4. The BET surface area decreases, appears to go through a minimum at50% treatment level and then increases.

5. The percent friability follows the same trend as the oil absorption.The friability of silica increases with increase in the post-treatmentlevel and goes through a maximum friability at 30% treatment level.

It is clear from the data listed in Table II that improvements whichresult from post-treatment of the precipitated silica reaction slurryare unique and were truly unexpected.

The improved properties obtained by the practice of the instantinvention are depicted in FIGS. 1 through 5.

FIG. 1 is a graph illustrating the change in the structure index ofsilica as a function of silicate post-treatment.

FIG. 2 is a graph illustrating the change in the oil absorption ofsilica as a function of silicate post-treatment.

FIG. 3 is a graph illustrating the change in the void volume of silicaas a function of silicate post-treatment.

FIG. 4 is a graph illustrating the change in the BET surface area ofsilica as a function of silicate post-treatment

FIG. 5 is a graph illustrating the percent change in friability ofsilica as a function of silicate post-treatment.

In the practice of the invention it has been found that processingparameters can be varied in certain ranges as follows.

The process can be conducted at a reaction temperature between 60°C.-95° C. Ideal results were obtained when the reaction temperature of80° C. was used. This process cannot be conducted below 60° C. Below 60°C. a white, transparent, gel-like product is obtained which does notpossess suitable pigmentary properties.

The acid temperature can be varied between 32° C. to 65° C. Idealresults were obtained when the acid temperature was maintained at 38° C.

The silicate used for the post-treatment can be of the same or differentconcentration when compared with initial silicate used for acidulationto a final pH of 7. The silicate used for post-treatment may be addedgradually as a function of time or it can be metered into the reactionslurry as fast as possible.

The temperature of the silicate solution used for post-treating theprecipitated silica reaction slurry can be varied between 32° C.-75° C.Experiments led to the conclusion that an ideal temperature of silicateprior to treating the precipitated silica slurry should be 65° C.

While the prior art silica cannot be used for dentifrice application, itwas discovered that silicas produced by the instant improved processwere suitable for dentifrice use. For use in clear-gel dentifrices, therefractive index of the silica was controlled by up-grading to a valuebetween 1.45 to 1.46 by pre-mixing acid with an adduct material. Thesuitable adduct material is a solution of alum or aluminum sulfate,soluble calcium magnesium, and zinc compounds.

A series of experiments was run in which the adduct material waspre-mixed with the acid. For these tests, a stock solution of acid andadduct material was prepared by mixing 100 liters of 11.4% acid with 7liters of 15% aluminum sulfate solution. This mixed acid-alum solutionwas used for acidulation of silicate. The following experiment were runsimilar to Example I through V except that acid contained a smallquantity of alum as specified above to produce controlled refractiveindex silicas.

    ______________________________________                                        EXAMPLES VI, VII, VIII, IX, & X                                               Post-Treatment                                                                Example No.                                                                            % Treatment                                                                              Volume Silicate Solution Required                         ______________________________________                                        VI        0         0           liters                                        VII      10         3.78        liters                                        VIII     30         11.35       liters                                        IX       50         18.92       liters                                        X        70         26.50       liters                                        ______________________________________                                    

Data obtained in Examples VI through X is listed in Table III. TableIIIA shows the effect of the adduct addition and post-treatment on thesurface area and refractive index values of silicas.

                                      TABLE III                                   __________________________________________________________________________    PROPERTIES OF CONTROLLED REFRACTIVE INDEX SILICAS                             AS A FUNCTION OF POST-TREATMENT LEVEL                                                                           BET                                         Example                                                                            % Treatment  Structure                                                                          Oil   Void Surface                                                                            Percent                                No.  Level  % WCM Index                                                                              Absorption                                                                          Volume                                                                             Area Friability                             __________________________________________________________________________    VI    0     85.6  595  185   3.03 239   0                                     VII  10     83.2  495  201   4.00 220  28                                     VIII 30     81.3  435  212   4.44 153  93                                     IX   50     78.7  370  205   3.67 185  78                                     X    70     77.9  350  193   3.19 198  45                                     __________________________________________________________________________

                                      TABLE IIIA                                  __________________________________________________________________________    EFFECT OF ADDUCT AND POST-TREATMENT ON PROPERTIES OF SILICA                          Silica W/Adduct                                                                            Silica W/O Adduct                                                                          Increase in Surface                                 Surface Area                                                                         Refractive                                                                          Surface Area                                                                         Refractive                                                                          Area Due to Adduct                           % Treatment                                                                          (Table III)                                                                          Index (Table II)                                                                           Index and Treatment                                __________________________________________________________________________     0     239    1.455 164    1.440 75                                           10     220    1.456 141    1.442 79                                           30     153    1.460 123    1.442 30                                           50     185    1.461 120    1.441 65                                           70     198    1.455 153    1.441 45                                           __________________________________________________________________________

The following conclusions can be drawn by examining data listed in TableIII.

1. The increase in the treatment level decreases the structure index ofcontrolled refractive index silicas of Examples VI through X.

2. The oil absorption values of controlled refractive index silicasincrease with the increase in the treatment level. The maximum oilabsorption was obtained at about 30% treatment level.

3. The particle-particle void volume of controlled refractive indexsilicas increased as a function of the treatment level. A maximumincrease was observed at a treating level of 30%.

4. The surface area decreased as a function of the treating level. Ofall the silicas in Examples VI through X, a minimum surface area wasobserved at 30% treatment level. Note that in general the surface areaof silicas in Examples VI through X is higher than the correspondingcounterparts in Example I through V. This increase in surface area isdue to the presence of a small amount of an adduct material which waspre-mixed with the acid prior to producing controlled refractive indexsilicas of Examples VI through X. It is critical that the adductmaterial is pre-mixed with acid. If this is not done, the controlledrefractive index silicas cannot be prepared.

5. The percent friability of the controlled refractive index silicaincreases as the post-treatment level increases. A maximum friabilitywas observed at a level of 30% post-treatment.

The improved properties obtained (see Examples VI through X) in thepractice of the instant invention are depicted in FIGS. 6 through 10.

FIG. 6 is a graph illustrating how the structure index of the controlledrefractive index silicas changes as a function of the silicatepost-treatment.

FIG. 7 is a graph illustrating how the oil absorption of the controlledrefractive index silicas changes as a function of the silicatepost-treatment.

FIG. 8 is a graph illustrating how the void volume of the controlledrefractive index silicas changes as a function of the silicatepost-treatment.

FIG. 9 is a graph illustrating how the BET surface area of thecontrolled refractive index silicas changes as a function of thesilicate post-treatment.

FIG. 10 is a graph illustrating how the percent friability of thecontrolled refractive index silica changes as a function of the silicatepost-treatment.

EFFECT OF SILICATE COMPOSITION, EXAMPLES XI-XXII

To further study the improvements of the instant invention, silicates ofdifferent compositions and concentration were used. The reaction slurryafter post-treatment was treated with silicate solutions whichcorresponded to 20% conditioned level. Three different silicatesolutions designated "A", "B", and "C" were used for this study.

Silicate Solution "A" had a composition of 3.77% Na₂ O, 12.23% SiO₂, andspecific gravity of 1.139.

Silicate Solution "B" had a composition of 3.30% Na₂ O, 10.7% SiO₂, anda specific gravity of 1.120.

Silicate Solution "C" had a composition of 2.82% Na₂ O, 9.18% SiO₂, anda specific gravity of 1.101.

In each case a control silica (without treatment) was prepared similarto teachings of Example I. Post-treated silicas were prepared followingthe procedures of Example II. Data is summarized in Table IV.

                                      TABLE IV                                    __________________________________________________________________________    SILICAS FROM DIFFERENT SILICATE SOLUTIONS                                                                    Oil   Void                                     Example                                                                            %     Silicate     BET Surface                                                                          Absorption                                                                          Volume                                   No.  Treatment                                                                           Solution                                                                           % WCM                                                                              SI Area, m.sup.2 /g                                                                     (cc/100 g)                                                                          (cc Hg/g)                                __________________________________________________________________________    XI*   0    A    84.4 540                                                                              200    195   3.33                                     XII  20    A    82.8 482                                                                              186    208   4.01                                     XIII*                                                                               0    B    85.1 570                                                                              208    206   3.09                                     XIV  20    B    83.6 510                                                                              200    221   3.92                                     XV*   0    C    86.8 656                                                                              247    175   2.57                                     XVI  20    C    85.0 567                                                                              226    187   2.71                                     __________________________________________________________________________     *Control experiments (without treatment).                                

The data of Table IV again substantiates that the post-treatment of thesilica reaction slurry as per the instant invention results in adecrease in the structure index and the surface area but an increase inthe oil absorption and the particle-particle void volume of silicas.This was truly unexpected.

In a further series of experiments, Examples XI through XVI wererepeated except that a calculated amount of an aluminum sulfate solution(as disclosed in Examples VI through X) was pre-mixed with the acid. Thealuminum sulfate solution was added to the acid in order to producesilica products having a controlled refractive index for use indentifrice compositions. Data obtained in the preparation of controlledrefractive index silicas is listed in Table V.

                                      TABLE V                                     __________________________________________________________________________    CONTROLLED REFRACTIVE INDEX SILICAS*                                                                         Oil   Void                                     Example                                                                            %     Silicate     BET Surface                                                                          Absorption                                                                          Volume                                   No.  Treatment                                                                           Solution                                                                           % WCM                                                                              SI Area, m.sup.2 /g                                                                     (cc/100 g)                                                                          (cc Hg/g)                                __________________________________________________________________________    XVII**                                                                              0    A    84.6 550                                                                              224    194   3.31                                     XVIII                                                                              20    A    82.2 462                                                                              174    219   3.98                                     XIX**                                                                               0    B    85.5 590                                                                              249    188   2.84                                     XX   20    B    82.8 482                                                                              210    221   3.44                                     XXI**                                                                               0    C    86.4 645                                                                              322    177   2.57                                     XXII 20    C    84.3 536                                                                              243    187   3.01                                     __________________________________________________________________________     *Refractive index of all silicas in Table V was 1.45-1.46.                    **Control experiments (without treatment)                                

By examination of the data in Table V, it will be seen that the surfacearea of controlled refractive index silicas is higher than theircorresponding counterparts listed in Table IV. This is thought to be dueto microporosity which is created in the silica particles by theaddition of adduct material. It is clear from the data of Table V thatcontrolled refractive index silicas wherein the precipitated silicareaction slurry is post-treated results in a decrease in structure indexand decrease in surface area, but an increase in oil absorption andparticle-particle void volume. This result, wherein the reduction in thestructure index without a corresponding reduction in the oil absorption,is truly unexpected. It is taught in the prior art that as the wet cakemoisture decreases, the oil absorption of a silica also decreases. Bythe practice of the instant invention, it is now possible to increasethe oil absorption of a pigment without increasing the wet cake moistureor structure index of silica. Stated differently, the instant inventionresults in a fantastically unique technique wherein it is now possibleto increase the oil absorption of a silicon dioxide without increasingthe wet cake moisture or production cost of a pigment. In fact, theprocess of the invention results in a silica pigment of higher oilabsorption and yet lower production costs.

Silica Containing Compositions

As noted above, the silicon dioxides of the instant invention can beused for various applications. As shown by FIGS. 1 through 10, silicaproducts produced by the practice of the instant invention haveparticularly and especially desirable physical and surface chemicalproperties. For use as a pigment in rubber, paper, paints, detergents,dentifrice, and conditioning applications, it is desirable that thesilica have a controlled particle size, surface area, oil absorption,refractive index, particle-particle void volume, and structure index. Asa consequence of the remarkable physical, chemical properties of thesilica pigments of the present invention, they are superior for use inrubber, paper, paint, detergent, dentifrice, and conditioning use ascompared with prior art silica products.

Rubber Compositions

The following materials, in the quantities and the manner indicated,describe a standard testing composition employed to test the products ofthe invention in rubber for use in shoe soles, heels, and the like:

    ______________________________________                                        Test recipe - rubber in shoe soles and heels:                                                         Parts by wt./100                                      ______________________________________                                        (1) Styrene-butadiene rubber (Plioflex                                        1778-SBR, nondiscoloring low temperature                                      polymer containing 37 parts light color                                       naphthenic oil per 100 parts cold rubber-                                     Mooney viscosity 42-45) 42.8                                                  (2) Styrene-butadiene rubber (Plioflex                                        1510-white, solid low temperature                                             cold rubber-Mooney viscosity of 29-36)                                                                35.0                                                  (3) Styrene-butadiene rubber (Plioflex                                        1950-white, friable mixture of 50% low                                        temperature SBR containing 37 parts of                                        naphthenic oil and 50% high styrene                                           resins)                 93.6                                                  (4) Zinc oxide          6.6                                                   (5) Zeolex® 23 (synthetic pigment material                                produced according to U.S. Pat. No.                                           2,739,073)              7.0                                                   (6) Pigment (products of Examples I through                                   V herein)               70.0                                                  (7) Stearic acid        1.0                                                   (8) Carbowax-(polyglycol-6000                                                 molecular weight)       4.0                                                   (9) Phthalic anhydride  .65                                                   (10) NOBS Special (N-oxydiethylene                                            benzothiazole-2-sulfenamide)                                                                          1.00                                                  (11) Captax (mercaptobenzothiazole)                                                                   .80                                                   (12) DOTG (diorthotolylguanidine)                                                                     .80                                                   (13) Octamine (diphenylamine and                                              diisobutylamine)        1.0                                                   (14) Circo light oil (naphthenic type oil)                                                            15.0                                                  (15) Sulfur             2.8                                                   ______________________________________                                    

The following materials, in the quantities indicated, describe astandard testing composition employed to test the exemplary productsherein in rubber for use in tires, more particularly, heavy-duty tiresof the off-the-road type:

    ______________________________________                                        Test recipe - off-road-tires:                                                                          Parts by wt.                                         ______________________________________                                        (1) Rubber - (Natural smoked sheets)                                                                   100.0                                                (2) Carbon black (ISAF-intermediate                                           super abrasion furnance black-                                                J. M. Huber Corporation,                                                      Borger, Texas            37.0                                                 (3) Pigment (end product of examples herein)                                                           20.0                                                 (4) Zinc oxide           5.0                                                  (5) Stearic acid         3.0                                                  (6) 6-dodecyl-1,2-dihydro-2,2,4-trimethyl                                     quinoline (Santoflex DD) 0.5                                                  (7) Polymerized 1,2-dihydro-2,2,4-trimethyl                                   quinoline (Flectol H)    1.5                                                  (8) Pine tar             5.0                                                  (9) Terpene resin acid blend                                                  (Turgum S)               2.0                                                  (10) 2,2'-Benzothiozyl disulfide (MBTS)                                                                0.8                                                  (11) Sulfur              2.8                                                                           177.6                                                ______________________________________                                    

The silica pigments of Examples I through V and a commercial pigmentHi-Sil 233 (a product of PPG Industries, Inc.) were incorporated intothe above-mentioned shoe heels and soles formulation and subjected tovarious conventional tests.

The physical tests and results are reported in Table VI.

                                      TABLE VI                                    __________________________________________________________________________                Rubber Data, Silica of Examples I through V                                   Ex. I                                                                            Ex. II                                                                            Ex. III                                                                           Ex. IV                                                                            Ex. V                                                                             Hi-Sil 233                                     __________________________________________________________________________    95% Cure, min                                                                             4.25                                                                             4.58                                                                              4.08                                                                              4.16                                                                              4.33                                                                              4.92                                           Scorch time, min                                                                          1.33                                                                             1.67                                                                              1.42                                                                              1.42                                                                              1.42                                                                              1.80                                           Tensile strength, p.s.i.*                                                                 2200                                                                             2075                                                                              1875                                                                              1900                                                                              1908                                                                              2142                                           Modulus, 300% p.s.i.*                                                                      875                                                                              926                                                                              1042                                                                               983                                                                               941                                                                               825                                           Modulus, 400% p.s.i.*                                                                     1175                                                                             1250                                                                              1325                                                                              1292                                                                              1242                                                                              1100                                           Elongation, percent*                                                                       608                                                                              573                                                                               533                                                                               542                                                                               550                                                                               625                                           Shore A Hardness*                                                                         85 88  86  86  88  86                                             NBS Abrasion*                                                                             62 75  70  70  65  63                                             __________________________________________________________________________     *8 minute cure                                                           

From Table VI, it can be seen that the rubber compositions incorporatingsilica pigment prepared by the improved process of the instant inventionhave much higher modulus and abrasion resistance values than controlsilica of Example I and the reference standard Hi-Sil 233. Thesedesirable properties of the instant invention make the rubbercompositions useful for shoe heels and soles, tire treads and carcasses,engine mounts and belts.

FIG. 11 illustrates the change in modulus values of rubber compositionscomprising silica pigments of Examples I through V as a function of thepercent silicate post-treatment. Thus, examining Table VI, it is clearthat silica pigments of instant invention exhibit superior rubberreinforcing properties. See FIG. 11, line A for Hi-Sil 233-prior artproduct.

The rubbers (alternatively referred to herein as elastomers whichmaterials are unvulcanized) which can be employed in the inventioninclude both natural and synthetic rubbers. Exemplary of suitablesynthetic rubbers are styrene-butadiene, butyl rubber, nitrile rubber,polybutadiene, polyisoprene, ethylene propylene, acrylic, fluorocarbonrubbers, polysulfide rubbers, and silicone rubbers. Mixtures orcopolymers of the above synthetic rubbers can be employed alone or incombination with natural rubber. The preferred rubbers are nitrilerubber, styrene-butadiene rubber, natural rubber, polyisoprene, andmixtures because they are most compatible with polyester fibers althoughminor amounts of other rubbers can be included without adverse effects.

Paper Compositions Containing Silicas

The ease with which material printed on one side of a sheet can be seenthrough on the other is a print quality item that has been the subjectof complaints, generally in the field of newspaper printing. This effectis frequently called "print show-through" or "print-through." It isalways important to improve the printability of newsprint becauseprintability is becoming increasingly more important to newspapers,their readers, and advertisers.

Silicas of Examples I through X were evaluated in a newsprintapplication to determine if silicas of the instant invention willimprove the printability of newsprint. Data of silica-filled newsprintis listed in Table VII.

                                      TABLE VII                                   __________________________________________________________________________    NEWSPRINT EVALUATION OF SILICAS OF EXAMPLES I THROUGH X                       Example                                                                            %   Basis                                                                             Caliper                                                                           TAPPI TAPPI                                                                             S/T* at                                                                             S/T*                                         No.  Filler                                                                            Weight                                                                            (mils)                                                                            Brightness                                                                          Opacity                                                                           2 g/m.sup.2 Ink                                                                     % Reduction                                  __________________________________________________________________________    Unfilled                                                                           None                                                                              32.2                                                                              3.3 60.0  86.1                                                                              13.8  --                                           I    2   32.4                                                                              3.3 60.3  86.2                                                                              8.3   40                                                4   32.8                                                                              3.3 60.5  86.3                                                                              5.0   64                                           II   2   31.9                                                                              3.3 60.5  86.3                                                                              8.6   38                                                4   32.2                                                                              3.3 60.8  86.4                                                                              5.3   61                                           III  2   31.5                                                                              3.2 60.7  86.3                                                                              8.1   41                                                4   31.7                                                                              3.2 61.3  86.4                                                                              4.7   66                                           IV   2   31.7                                                                              3.2 60.7  86.5                                                                              7.8   44                                                4   32.2                                                                              3.3 61.3  86.8                                                                              4.4   68                                           V    2   30.8                                                                              3.2 60.8  86.7                                                                              7.5   46                                                4   32.7                                                                              3.3 61.5  87.2                                                                              4.0   71                                           VI   2   31.9                                                                              3.2 60.4  86.2                                                                              8.4   39                                                4   32.3                                                                              3.3 60.6  86.3                                                                              5.3   62                                           VII  2   32.2                                                                              3.3 60.5  86.4                                                                              7.4   47                                                4   32.5                                                                              3.3 60.9  86.5                                                                              3.9   72                                           VIII 2   31.8                                                                              3.2 60.7  86.4                                                                              7.5   46                                                4   31.9                                                                              3.2 61.4  86.6                                                                              4.0   71                                           IX   2   32.0                                                                              3.2 60.9  86.7                                                                              7.6   45                                                4   32.3                                                                              3.3 61.5  87.2                                                                              4.2   70                                           X    2   32.0                                                                              3.2 60.9  86.5                                                                              8.4   39                                                4   32.0                                                                              3.2 61.5  86.8                                                                              5.1   63                                           __________________________________________________________________________     *S/T = Strike Through                                                    

Examination of data in Table VII reveals that silicas of the instantinvention increase the brightness, opacity, and print quality of thenewsprint. It is clear that silicas of the instant invention when usedas fillers increase the strike-through reduction. This is a beneficialproperty to have for improving the printing properties of newsprint.Thus, silicas when incorporated into the fibers of newsprint result inincreased strike-through reduction, brightness, and opacity.

Increase in the brightness and opacity of the newsprint may beattributed to changes in the light scattering properties and increase insurface area which result from the addition of silicas of Examples Ithrough X to the newsprint.

Brightness is increased in the blue region of the spectrum. The resultis a whiter, brighter newsprint which has greater contrast with theprint.

The combination of reduced migration of the ink vehicle, increasedopacity and brightness increases the contrast ratio of printed andunprinted pages and results in sharper half tones of better printquality.

Experiments have indicated that silicas are bonded to the fibrils anddistributed through the capillaries between fibrils. In essence, asilica pigment-fiber complex is formed which has significantly differentphysical and chemical properties from the original fiber.

The following test procedures were used in evaluating the newsprintproperties of silicas pigment:

The unfilled furnish was 65 percent groundwood, 35 percent kraft, 0.0002percent crystal violet, and sufficient alum to adjust the pH to 4.5.

Handsheets containing ash levels of 2 and 4 percent were formed in an8-in. sheet mould equipped with an 80-mesh wire. The sheets were pressedin a Williams press and airdried overnight in 8-in. by 8-in. dryingframs at 73° F. and 50 percent R.H. The sheets were calendered to acaliper of 0.0032 in. and a Bausch and Lomb gloss of about 12 percent.

Optical Testing of the Handsheets

The sheets were evaluated for brightness and opacity according to TAPPIStandards T452 m-58 and T425 m-60 using the Standard Brightness Testerand a B & L Opacimeter.

Physical Testing of the Handsheets

Printing tests were made at standard conditions of temperature andhumidity on a Universal No. 1 Vandercook proof press using a standardnewsprint ink and a newsprint plate mounted type high. The plateconsisted of a series of half tones and a solid area (31/2 in. by 3 in.)which was used to measure the print response. Prints were made with 4mils impression by press bed adjustment and the ink pick-up determinedby weighing the sheets before and after printing.

The printing evaluations were made at an ink pick-up equivalent to 2.0g./m.² for the solid portion; the ink pick-up for the solid area inproportion to the whole print was predetermined by experiment. Although1.75 ml. of ink applied to the press distribution system produced an inkpick-up near 2.0 g./m.², slight variations in ink pick-up necessitatedprinting each ash level at three ink levels (1.5, 1.75, 2.0 ml.). Theprinting value at exactly 2.0 g./m.² was graphically obtained byplotting print values against the actual ink pick-up. Likewise, the ashcontent varied somewhat so it was necessary to print sheets containingabout 2 and 4 percent ash so that comparative values at a given ashcontent could be obtained by plotting print response against ashcontent.

Printed sheets were allowed to condition overnight at 73° F. and 50percent R.H. (relative humidity).

The printing intensity and strike-through were evaluated by the StandardBrightness Tester at 457 mμ and determined in accordance with Larocque'sequation: ##EQU3## The printed side was used to determine printingquality or color intensity and the reverse or unprinted side fordetermining strike-through or the degree of ink penetration.

Detergent Compositions

Typical home-laundry detergents consist of the following ingredients:

    ______________________________________                                        Ingredient         Percent, by Weight                                         ______________________________________                                        Sodium Tripolyphosphate                                                                          12-50                                                      Surface Active Agents                                                                            10-20                                                      Liquid Sodium Silicate                                                                           5-8                                                        Soil Redeposition Agents                                                                         0.5-1.5                                                    Fluorescent Dyes   0.05-1                                                     Water               2-12                                                      Sodium Sulfate     Balance                                                    ______________________________________                                    

Surface active agents mainly consist of anionic linear alkyl benzencsulfonate (LAS) and non-ionic alcohol based ethoxylates (AEO).Surfactant is needed in detergent to extend the functional performanceof a detergent builder.

Non-ionic surfactants are added at a level of 4-6% (typical non-ionicsurfactants currently being used are Shell's Neodol 25-7 and 45-11) tothe other ingredients of detergent compositions. The resulting slurry isspray dried. Non-ionic surfactants contain small fractions ofshort-chain molecules call "light ends." During the spray drying step,the "light ends" do not incorporate into the finished detergent bead andgo out of the dryer exhaust and result in a white cloud referred to as"plume."

Detergent producers are anxious to cut down this "plume" and severalmechanical advances have been made to scrub the stack gases butscrubbing process is not 100% effective. Also, the equipment required toclean the stack gases is very expensive.

We have found an inexpensive solution to the problem in which silicas ofthe present invention can be used to convert the liquid non-ionicsurfactants to dry free flowing particulate form so that dried-upsurfactant can be post added to the spray dried detergent formulation.Thus, precipitated silica pigments of the instant invention are usefulfor drying-up non-ionic surfactants in the free flowing form. Thus,silica pigments can be used in the detergent compositions to solve anair pollution problem called "pluming."

Neodol 25-9 surfactant (manufactured by Shell Chemical Company) wasdried up by using silica pigments as the carriers or adsorbents. Themaximum amount of Neodol that can be dried up on silica is listed inTable VIII.

                  TABLE VIII                                                      ______________________________________                                        DRYING-UP NEODOL 25-9 ON PRECIPITATED                                         SILICA PIGMENTS                                                               Silica of    Flow Time     % Active                                           Example No.  (seconds)     Surfactant                                         ______________________________________                                        I*           36            61.2                                               II           24            65.9                                               III          15            70.0                                               IV           17            70.0                                               V            18            70.9                                               VI*          34            62.1                                               VII          21            70.4                                               VIII         17            71.2                                               IX           20            69.0                                               X            25            67.0                                               ______________________________________                                         *Controls                                                                

From data in Table VIII above it is clear that silicas of Examples IIthrough V and Examples VII through X exhibit superior flow propertiesand drying capacity when compared with the corresponding control silicasof Examples I and VI.

Thus, the method of drying up non-ionic surfactants results in superiorfree flowing surfactant powders. These surfactant powders can beefficiently used by post-adding to detergent compositions. Thus, silicasof the instant invention are useful in detergent compositions and thesesilica pigments impart superior properties which help in solving animportant air pollution problem. Other prior art silicas may be useful,but all such silicas are either very expensive or not efficient enoughto be used in detergent compositions.

Pharmaceutical and Cosmetic Compositions

As a vehicle for liquid pharmaceutical preparations, polyols are usedextensively and these polyols offer many unique advantages for syrups,elixirs, and other liquid pharmaceutical and cosmetic formaulations.

Sorbitol and glycerine are widely used as humectants in pharmaceuticaland cosmetic preparations. Sorbitol is commercially available from ICI,U.S.A. in a 70% solution under the trademark "Sorbo." Sorbo is a sugaralcohol, C₆ H₈ (OH)₆ which occurs in nature as a nutritive ingredient ofmany fruits and berries. Sorbitol, chemically is a hexahydric member ofthe polyhydric alcohol or polyol family, of which glycerine is thetrihydric member.

The silica pigments of the instant invention can be efficiently used ina variety of cosmetic products where a thickener, suspending agent,emulsion stabilizer, emulsification aid, binder, or a viscosity buildingagent is required.

The efficiency with which silica pigments of the instant invention canbe used in drying up humectants can be seen by examining data of TableIX. For use in cosmetics, the silica pigments of Examples I through Vwere air milled using a fluid energy mill and then incorporated invarious humectants.

Humectant A: This solution was prepared by mixing 45 parts of sorbitolsolution with 15 parts of glycerine.

Humectant B: This solution was prepared by mixing 30 parts of sorbitolsolution with 20 parts of glycerine.

Humectant C: This solution was prepared by mixing 20 parts of sorbitolwith 30 parts of glycerine.

Humectant D: This solution was prepared by mixing 15 parts of sorbitolwith 30 parts of glycerine.

                  TABLE IX                                                        ______________________________________                                        % ACTIVE HUMECTANT /100 g SILICA                                              Silica of                                                                              Humectant Humectant Humectant                                                                             Humectant                                Example No.                                                                            A         B         C       D                                        ______________________________________                                        I        157       165       175     180                                      II       230       280       250     220                                      III      250       300       320     250                                      IV       220       250       210     280                                      V        210       200       220     200                                      ______________________________________                                    

The silica pigments of the instant invention exhibit superior viscositybuilding and carrying capacity than the control silica of Example I.

The viscosity building data of silicas from Example I through V inHumectant A is given in Table X.

                  TABLE X                                                         ______________________________________                                        VISOCITY BULIDING PROPERTIES IN HUMECTANT A                                   Silica From                                                                            Percent Loading of Silica in Humectant A                             Example No.                                                                            2.5%        5.0%        10%                                          ______________________________________                                        I        350 CPS     400 CPS      850 CPS                                     II       400 CPS     750 CPS     2000 CPS                                     III      350 CPS     500 CPS     2850 CPS                                     IV       325 CPS     450 CPS     1200 CPS                                     V        338 CPS     500 CPS     2250 CPS                                     ______________________________________                                    

The viscosity data was run by using Brookfield Viscometer.

Paint Coating Composition

The silica pigments of the instant invention were air milled using fluidenergy mill and then incorporated in a paint system for reduction ingloss of a paint system.

In order to provide protection and to produce a pleasing appearance, avariety of surfaces, such as wood, metal, fabric, paper, or plastics,are coated with clear flatting compositions containing dispered orsuspended particles of a flatting agent which reduces the gloss or sheenof the coating and the coated substrate, preferably withoutsubstantially reducting the transparency of the flat coating. Forexample, wood finishes which serve to protect the surface againstabrasion and strain, yet do not conceal the beauty of the grain, aremade to simulate desirable hand-rubbed finishes by incorporatingflatting agents therein which normally are dispersed fine particles ofsuch materials as silicas. The best effects are obtained with silicas ofuniform particle size down to the submicron range. Small size anduniformity are necessary to achievwe a smooth coating without whitespecks or without a graying effect which would detract from theappearance of the coating.

For paint flatting application, 10 grams of silica (which was airmilled) of the instant invention was mixed with 350 grams of thenitrocellulose lacquer (comforming to Military specificationMIL-L-10287A--amendment 2, Type II, of issue 27, August 1959) and mixedfor 3 minutes using the low speed setting of the Hamilton-Beach #30mixmaster. The lacquer containing dispersed silica was tested for Hegmanfineness of grind (5.50) and cleanliness of grind.

The lacquer containing dispersed silica from Examples II through V wasmixed with no lacquer and additional lacquer to prepare stock solutioncontaining 10%, 3.5%, and 1.75% by weight of vehicle solids. A drawdownof various stock solutions (containing 10%, 3.5%, and 1.75% silica islacquer) was made on carrara glass using a 34 wire coatings applicationrod. Carrara glass drawdowns were allowed to dry for 45 minutes underdust-free conditions. Using the above method, drawdowns were also madefrom stock solutions containing the silica developed via the prior artprocesses of Example I.

Using the Gardner multi-angle gloss meter, the gloss and sheen values ofthe various drawdowns were measured at 60° and 85°, respectively. Thesevalues were compared with measured values obtained when a prior artsilica was dispersed in the lacquer.

Silicas of the present invention result in cleaner Hegman grinds andexhibit better clarity when dispersed in the lacquer. The better clarityis attributed to the fact that the silicas of the present invention areof uniform particle sizes and favorable structures.

Flatting data listed in Table XI suggests that the novel silicas of thepresent invention exhibit lower gloss and sheen values than the control,Example I. Lower gloss and sheen values are preferred and advantageousfor paint flatting applications.

                  TABLE XI                                                        ______________________________________                                        PAINT FLATTING EVALUATION                                                     Silica From                                                                            60° Gloss                                                                              85° Sheen                                     Example No.                                                                            10%     3.5%    1.75% 10%   3.5%  1.75%                              ______________________________________                                        I        10      38      54    37    72    82                                 II       8       28      27    26    59    71                                 III      6       20      31    20    30    51                                 IV       5       22      29    18    29    57                                 V        7       28      31    15    17    39                                 ______________________________________                                    

Examining data of Table XI, it is clear that post-treated silicas,Example II through V, exhibit superior properties than the control ofExample I (no post-conditioning).

Dentifrice Composition

The silica pigments of the instant invention can be efficiently used asthickening agent in dentifrices. Where a controlled refractive indexthickener is required, this property can be controlled by the additionof small amount of an adduct material as illustrated in Examples VIthrough X. Controlled refractive index silicas (see Example VII throughX) exhibit superior thickening properties in a clear gel toothpaste thanthe control of Example VI.

If the pigments of the invention are used in toothpaste compositions,the dentifrice (if in the form of a paste) may contain humectantmaterials and binders to give the dentifrice a smooth texture and goodflowability. Glycerine, sorbitol, corn syrup, glucose, and the like maybe used as carriers. Examples of binders include gum tragacanth, sodiumcarboxymethylcellulose, and the like. The above materials, as well asthe specific formulation of the toothpaste, are well known in the artand are disclosed, for example, in U.S. Pat. Nos. 2,994,642 and3,538,230 and numerous publications.

As discussed above, the unique silicas of the invention may beadvantageously employed as thickening and polishing agents in toothpastecompositions. This is truly remarkable inasmuch as precipitated silicasof the prior art cannot be so employed. If the products of the inventionare used in toothpaste compositions, and as known in the art, thedentifrice may contain, e.g., humectant minerals and binders to give thedentifrice a smooth texture and good flowability. A detailed disclosureof dentifrice formulations is given in U.S. Pat. No. 3,729,961.

In this regard, dentifrice formulations have been produced, ranging fromliquids and powders to the highly popular pastes or dental creams.Dental creams are the more difficult to compound successfully in thatthey require careful balancing of polishing agent, humectant, water,binder, preservatives, detergents, flavoring, sweeteners, andtherapeutic agents to produce a smooth homogeneous paste.

Most modern dental cream formulations use one of several phosphatematerials as the polishing agent. Examples of the phosphate polishingagents are dicalcium phosphate, anhydrous dicalcium phosphate,tricalcium phosphate, thermally converted dicalcium phosphate, andinsoluble sodium metaphosphate. The amount of phosphate materials addedto the dental formulations will range between about 5 percent and 60percent by weight.

The most widely used humectants in toothpaste are glycerine andsorbitol. Propylene glycol is also used in small amounts and to a verylimited extent. The primary function of humectant as part of the liquidphase is to retain moisture which provides good texture and maintains anattractive glossy appearance when the paste is exposed to air.

The binder employed therein is to prevent separation of the liquid andsolid phases. The most conventionally used binders are the seaweedcolloids and synthetic derivatives of cellulose, specificallycarragcenan and sodium carboxymethyl cellulose. Others, such as gums,have been used. Combinations of these binders have also been employed.

Since the natural and synthetic water dispersions of organic binders aresubjected to microbial or mold attack, a relatively small amount ofpreservatives is added to the paste. Examples of preservatives used inthe industry are the esters of parahydroxyl benzoates.

The function of the detergents within the dental formulation is toprovide greater cleansing action due to the lowering of the surfacetension and the sudsing action in the mouth. Among detergents used aresodium N-lauryl sarcosinate, sodium lauryl sulfate, sulfoculaurate,sodium alkyl sulfoacetate, and sodium dioctyl sulfosuccinate.

Since toothpaste flavoring probably represents the greatest singlefactor in consumer acceptance, great care has been employed in selectingbalanced blends of different essential oils. These are rarely, if ever,used alone. Combinations of principal flavors are wintergreen,peppermint, and sassafras, and are used with secondary oils such aspimento, clove, and anise.

Saccharin and sodium cyclamate are widely used to improve taste andenhance the flavor qualities of the toothpaste. The synthetic sweetenersmay be used in combination to obtain optimum sweetness and absence ofafter-taste. Their desirable properties are obtained at very lowconcentrations and consequently they have negligible influence on thetoothpaste consistency.

Since water is such a common element, it is important in obtainingstable toothpaste formulations to employ substantially pure watertherein. It is common practice to demineralize the water that isemployed.

The therapeutic agents within the dental creams are to prevent decay ofthe tooth and are commonly in the form of stannous fluorides and sodiumfluoride material.

Difficulties have been encountered in using combinations of the abovematerials in modern dentifrice formulations. Specific scavenging of thefluoride ions by the phosphate and calcium containing polishing agentshave been experienced. Thus, in formulating a dentifrice composition, aplishing agent must be selected to provide excellent polishingproperties and have a very high degree of compatibility with thefluoride system and an particular should not scavenge the fluoride ion.

The silica pigments when prepared by the improved process disclosed inthe instant invention are suitable for use in dentifrices as athickener. While the prior art silicas are not suitable due tocontrolled physical-chemical properties and due to control of refractiveindex, the silicas of the instant invention are useful thickeners inclear-gel and opaque dentifrices.

It is disclosed in the literature that conventional syntheticprecipitated silicas are unsuitable as polishing and abrasive agents intoothpaste compositions. See German Pat. No. 974,958; French Pat. No.1,130,627; British Pat. No. 995,351; Swiss Pat. No. 280,671; and U.S.Pat. No. 3,250,680. In this regard, it is disclosed in U.S. Pat. No.3,538,230 that known amorphous silicas, such as precipitated silicas,pyrogenic silicas, and aerogels, are unsuitable for dentifrice usebecause they show substantially no cleaning ability on human teethbecause of their initial small particle size and because of the ease inwhich they break down into small particle sizes which result in poorcleaning ability.

Further, and in more detail, conventional silicas and amorphousprecipitated alumino silicates cannot be used for a clear-gel toothpastebecause of their high refractive index (1.55) and because they lack theneeded thickening, polishing characteristics when added to thetoothpaste base composition. Clear-gel toothpaste contains a highpercentage of abrasive and polishing agent in the toothpaste formula.The major function of the abrasive and polishing agent is to removestains, food debris, and bacterial plaque from the human tooth surface.Ideally the polishing agent should provide a maximum cleaning action atacceptable abrasion levels and must be compatible at high loadings of15% up to 50% with other toothpaste formula ingredients.

In an excellent chapter on Dentifrices in Sagarin's book, Cosmetics:Science & Technology, Gershon and Morton Pader have reviewed severaldentifrice formulations. We have found that controlled refractive indexsilicas of Examples VII through X are useful thickeners when formulatedin a clear-gel dentifrice formulation No. 8 listed on page 500 ofSagarin's book. The dentifrice formulation No. 8 listed on page 500consists of the following ingredients:

    ______________________________________                                        Dehydrated silica gel  14.00%                                                 Silica aerogel         7.50%                                                  Sodium carboxymethylcellulose                                                                        0.60%                                                  Sorbitol solution, 70% 67.82%                                                 Glycerol               5.74%                                                  Sodium lauryl sulfate  1.26%                                                  Color, flavor          2.77%                                                  Sodium hydroxide solution, 30%                                                                       0.31%                                                  ______________________________________                                    

In the above formulation, silica aerogel was substituted by silicas ofthe instant invention and acceptable thickening properties were impartedby these unique silicas of Examples VII through X.

Silicas of Examples II through V and VII through X can also be used asefficient thickeners in opaque dentifrices.

What is claimed is:
 1. A dentifrice including, as a thickening andpolishing agent, a finely divided, amorphous, precipitated silicondioxide having the combined properties of a structure index of less than505 and an oil absorption of greater than 190 cc/100 grams; said silicondioxide comprising not less than 90% SiO₂ and including a metal cationincorporated therein, said metal cation serving to control therefractive index of said silicon dioxide such that said refractive indexis not less than 1.45 and serving to control the surface area of saidsilicon dioxide such that said surface area is not less than 153 m²/gram, said metal cation being selected from the group consisting ofaluminum, magnesium, zinc, and calcium.
 2. A dentifrice including, as athickening and polishing agent, a finely divided, amorphous,precipitated silicon dioxide having a wet cake moisture content of frombetween about 79.8 to 83.5%; a structure index of from between about 359to 505; an oil absorption of from between about 190 to 202 cc/100 grams;a void volume of from between about 3.55 to 4.14 cc Hg/gram SiO₂ ; a BETsurface area of from between about 120 to 153 m² /gram; and a percentfriability of up to about 98%.
 3. A dentifrice including, as athickening and polishing agent, a finely divided, amorphous,precipitated silicon dioxide having a wet cake moisture content of frombetween about 77.9 to 83.2%; a structure index of from between about 365to 495; an oil absorption of from between about 193 to 212 cc/100 grams;a void volume of from between about 3.19 to 4.40 cc Hg/gram SiO₂ ; a BETsurface area of from between about 153 to 220 m² /gram; and a percentfriability of up to about 93%; said silicon dioxide comprising at least90% SiO₂ and having incorporated therewith a metal cation selected fromthe group consisting of aluminum, magnesium, zinc, and calcium tothereby control the surface area thereof and to increase the refractiveindex thereof to a value of at least 1.45.
 4. A dentifrice compositionaccording to claim 1 in a form of a toothpaste which comprises saidthickening and polishing agent together with humectants, binders,preservatives, detergents, flavoring agents, sweeteners and therapeuticagents.
 5. A toothpaste according to claim 4 wherein said humectant isselected from the group consisting of glycerine, sorbitol and mixturesthereof, and wherein said binder is selected from the group consistingof seaweed colloids, synthetic derivatives of cellulose, gums andmixtures thereof.
 6. A dentifrice composition according to claim 2 in aform of a toothpaste which comprises said thickening and polishing agenttogether with humectants, binders, preservatives, detergents, flavoringagents, sweeteners and therapeutic agents.
 7. A toothpaste according toclaim 6 wherein said humectant is selected from the group consisting ofglycerine, sorbitol and mixtures thereof, and wherein said binder isselected from the group consiting of seaweed colloids, syntheticderivatives of cellulose, gums and mixtures thereof.
 8. A dentifricecomposition according to claim 3 in a form of a toothpaste whichcomprises said thickening and polishing agent together with humectants,binders, preservatives, detergents, flavoring agents, sweeteners andtherapeutic agents.
 9. A toothpaste according to claim 8 wherein saidhumectant is selected from the group consisting of glycerine, sorbitoland mixtures thereof, and wherein said binder is selected from the groupconsisting of seaweed colloids, synthetic derivatives of cellulose, gumsand mixtures thereof.
 10. A clear gel toothpaste composition containinghumectants and binders in the liquid phase, and thickening and polishingagents, said composition including, as a thickening and polishing agent,a finely divided, amorphous, precipitated silicon dioxide having thecombined properties of a structure index of less than 505 and an oilabsorption of greater than 190 cc/100 grams; said silicon dioxidecomprising not less than 90% SiO₂ and including a metal cationincorporated therein, said metal cation serving to control therefractive index of said silicon dioxide such that said refractive indexis not less than 1.45 and serving to control the surface area of saidsilicon dioxide such that said surface area is not less than 153 m²/gram, said metal cation being selected from the group consisting ofaluminum, magnesium, zinc, and calcium; wherein the refractive index ofsaid liquid phase is substantially the same as said silicon dioxide. 11.A clear gel toothpaste composition containing humectants and binders inthe liquid phase, and thickening and polishing agents, said compositionincluding, as a thickening and polishing agent, a finely divided,amorphous, precipitated silicon dioxide having a wet cake moisturecontent of from between about 79.8 to 83.5%; a structure index of frombetween about 359 to 505; an oil absorption of from between about 190 to202 cc/100 grams; a void volume of from between about 3.55 to 4.14 ccHg/gram SiO₂ ; a BET surface area of from between about 120 to 153 m²/gram; and a percent friability of up to about 98%; wherein therefractive index of said liquid phase is substantially the same as saidsilicon dioxide.
 12. A clear get toothpaste composition containinghumectants and binders in the liquid phase, and thickening and polishingagents, said composition including, as a thickening and polishing agent,a finely divided, amorphous, precipitated silicon dioxide having a wetcake moisture content of from between about 77.9 to 83.2%; a structureindex of from between about 365 to 495; an oil absorption of frombetween about 193 to 212 cc/100 grams; a void volume of from betweenabout 3.19 to 4.40 cc Hg/gram SiO₂ ; a BET surface area of from betweenabout 153 to 220 m² /gram; and a percent friability of up to about 93%;said silicon dioxide comprising at least 90% SiO₂ and havingincorporated therewith a metal cation selected from the group consistingof aluminum, magnesium, zinc, and calcium to thereby control the surfacearea thereof and to increase the refractive index thereof to a value ofat least 1.45; wherein the refractive index of said liquid phase issubstantially the same as said silicon dioxide.